This invention relates generally to nonlinear semiconductor optical devices. More particularly, it relates to an ultrafast optically controlled optical switch and crossbar architecture for use in wavelength-division-multiplexed systems.
Communications systems are increasingly using optical fiber as the transmission medium, because of its low loss, immunity to interference, and extremely large bandwidth. In wavelength-division-multiplexed (WDM) systems, multiple wavelengths are used to allow many communication channels on a single optical fiber, allowing for much greater information transmission and network capacity. Modulated light beams are mixed into the fiber using optical couplers, and demultiplexed at the receiver end by optical filters. It is often necessary to transfer signals between optical networks operating at different wavelengths, and therefore transfer a particular optical signal from one channel to another. Switching information between channels requires the ability to change a particular signal of information from one wavelength to another. Switching of this manner requires both a device that can convert signal wavelengths and a system architecture, incorporating the device, that can be scaled to required capacities.
A WDM optical system is disclosed in U.S. Pat. No. 5,504,609, issued to Alexander et al. This system includes complex remodulators for transferring a signal from an input wavelength to an output wavelength. Each remodulator contains a photodiode or similar means for converting an optical input signal to an electrical signal, which is then amplified, filtered, and amplified again. The resultant electrical signal is used to modulate an optical source by exploiting the electro-optical effect in a waveguiding medium to create an amplitude-modulated output signal. The combination of electronic and optical elements required in the system of Alexander et al. greatly limit the net throughput in the system, and do not effectively take advantage of the increased bandwidth provided by the optical fiber. The remodulators also dissipate large amounts of power and make large arrays of switches impractical.
An all-optical wavelength converter is provided in U.S. Pat. No. 5,434,700, issued to Yoo. The device acts as a nonlinear optical mixer to combine an input signal with a pump signal to generate an output signal of a different wavelength. Specifically, the output frequency is the difference between the pump frequency and the input frequency. As is explicitly stated in the description, the pump frequency determines the frequency shift, and therefore the device cannot be used to convert multiple input channels to multiple output channels selectively. Instead, a separate device is required to convert between each input frequency and output frequency, requiring a set of parallel converters operating between neighboring WDM networks. Of course, this system cannot practically be scaled to WDM systems containing large numbers of channels. Furthermore, systems based on these techniques dissipate large amounts of power and are therefore not feasible for large-scale systems.
The technique employed by the device of Yoo, difference frequency generation, is used in a parametric wavelength interchanging cross-connect, described in U.S. Pat. No. 5,825,517, issued to Antoniades et al. The cross-connect of Antoniades et al. combines 2xc3x972 spatial optical switches with the wavelength converters of Yoo to allow arbitrary switching of signals among the channels of the WDM network. By selecting particular wavelengths of pump sources, the wavelength converters can be made to interchange signals between two channels in a single device. That is, each wavelength converter in the cross-connect takes two input signals with wavelengths xcex1 and xcex2, and produces two output signals of wavelengths xcex2 and xcex1, transferring the information carried in input signal xcex1 to output signal xcex2, and vice versa. Switching between systems with more than two channels requires complicated networks of 2xc3x972 spatial switches and wavelength converters. Because each wavelength converter is limited to a few predetermined frequencies, arbitrary switching requires a series of wavelength converters, each of which has a different pump frequency. In addition, the cross-connect of Antoniades et al. uses only a single set of WDM wavelengths for both input and output signals, and does not allow for truly arbitrary switching.
Optical switches for modulating optical signals have been disclosed in the prior art. These switches take advantage of the electroabsorption effect in devices that operate on picosecond time scales. A high-speed electro-optical modulator is disclosed in U.S. Pat. No. 4,525,687, issued to Chemla et al. This semiconductor device contains a multiple quantum well structure across which an electric field is applied. The applied electric field increases absorption for photon energies just below the band gap by the quantum-confined Stark effect (QCSE). As the electric field is increased further, the band edge shifts to lower photon energies. By carefully controlling an applied voltage, and therefore electric field, optical properties of the device can be changed at will. An optical signal with photon energy just below the band gap of the quantum well structure is absorbed or transmitted with just a small change in the applied voltage. Because this device is an electrically-controlled optical modulator, it cannot be used alone to provide the wavelength conversion required in WDM systems. The desired result can only be produced by combining this device with a photodetector for generating the required electrical signal in response to the optical signal. As with the system of Alexander et al., the combination is complicated, incurs high power dissipation, cannot operate at the required switching speeds, and is not easily integrated into arrays.
There is still a need, therefore, for a wavelength converting switch that can be used in an architecture that allows for a complete cross-connect, in which the signal from any input wavelength can be used to modulate the output at any wavelength, with multiple different wavelengths of data in and out of the cross-connect system.
Accordingly, it is a primary object of the present invention to provide an optical wavelength-converting cross-connect that allows for arbitrary switching of information between input and output signals.
It is a further object of the invention to provide a high-speed optically controlled optical switch for transferring signal information between an input light beam and an output light beam.
It is an additional object of the invention to provide a wavelength-converting switch that requires very low electrical and optical power inputs by exploiting the quantum-controlled Stark effect (QCSE).
It is another object of the present invention to provide an optical switch with controllable picosecond switching time scales.
It is an additional object of the present invention to provide an optical device that is easily integrated with the required electronics.
It is a further object of the present invention to provide an ultrafast gated photodetector that produces a signal requiring minimal processing.
These objects and advantages are attained by a semiconductor device for modulating an optical power light beam at a first wavelength with an optical signal light beam at a second wavelength. The device consists of two diodes: a detector diode, containing a detector absorbing layer for absorbing the optical signal beam; and a modulator diode, containing a modulator absorbing layer for absorbing the optical power beam. The modulator absorbing layer has an electric field-dependent absorption coefficient; the two diodes are in sufficient electrical communication that this coefficient is altered by absorption of the optical signal beam by the detector diode. Altering the coefficient modulates absorption of the optical power beam, and therefore transfers an information signal carried by the optical signal beam onto the optical power beam. Absorption of the optical power beam also generates a photocurrent in the modulator diode, and the device may also contain means for collecting the photocurrent, so that it acts as a gated photodetector. Preferably, the first and second wavelengths of the two light beams are substantially unequal.
Preferably, the optical power light beam propagates into a region of the modulator absorbing layer, and the optical signal light beam propagates into a region of the detector absorbing layer. These regions are within a short distance from one another, preferably less than the power beam diameter of the optical power beam, and also above one another. Preferably, this short distance is less than 20 xcexcm, and most preferably less than 5 xcexcm.
Preferably, the device contains means for applying a detector voltage to the detector diode and a modulator voltage to the modulator diode; most preferably, both diodes are reverse biased. The applied electric field shifts the absorption coefficient and enables absorption of the optical power light beam by the modulator absorbing layer. Preferably, the modulator absorbing layer is a quantum well or, more preferably, a plurality of quantum wells, allowing for absorption by the quantum-controlled Stark effect. Alternately, the modulator absorbing layer is a bulk semiconductor, allowing for absorption by the Franz-Keldysh effect.
The detector diode contains an upper contact layer and a lower contact layer, and the detector absorbing layer is positioned between these two contact layers. Similarly, the modulator diode contains an upper cladding layer and a lower cladding layer, and the modulator absorbing layer is positioned between these two cladding layers. The lower contact layer and upper cladding layer are in substantially planar parallel physical contact, providing the electrical communication of the device. Preferably, the upper contact layer and lower cladding layer have sufficiently high electrical conductivities that a voltage between the two is substantially constant, even as the voltage within the diodes changes as light beams are absorbed. Preferably, the upper cladding layer and lower contact layer have predetermined resistivities chosen to control the rate of diffusive electrical conduction in the layers. The resistances per square of the upper cladding layer and lower contact layer are preferably substantially larger than the resistances per square of the lower cladding layer and upper contact layer.
In a first embodiment, the lower contact layer and upper cladding layer are identical, and the upper contact layer and lower cladding layer are of the same semiconductor doping type. Preferably, the upper contact layer and lower cladding layer are n-type semiconductor material, and the upper cladding layer is p-type semiconductor material. Absorption of the optical signal beam by the detector diode creates electrical carriers that decrease the detector voltage and decrease the modulator voltage, altering the absorption coefficient and leading to decreased absorption of the optical power beam. In a second embodiment, the lower contact layer and upper cladding layer are of opposite semiconductor doping type. Preferably, the upper contact layer and upper cladding layer are n-type semiconductor material, and the lower contact layer and lower cladding layer are p-type semiconductor material. Absorption of the optical signal beam by the detector diode creates electrical carriers that decrease the detector voltage and increase the modulator voltage, altering the absorption coefficient and leading to increased absorption of the optical power beam.
Also provided in the present invention is an optical cross-connect for modulating a set of N optical power light beams with a set of N optical input signals to generate a set of M optical output signals. The optical input signals carry signal information that is transferred to the optical output signals, which are amplitude-modulated versions of the optical power beams. Preferably, the system is used for switching channels in a wavelength-division multiplexed system. In this case, the optical input signals are of different wavelengths from each other, and the optical power beams are of different wavelengths from each other, and different wavelengths from the optical input signals. The cross-connect consists of an array of the optically controlled optical switches described above, each of which is capable of modulating at least one power beam with at least one input signal to produce an optical output signal, so that the cross-connect can transfer signal information carried by each of the optical input signals to each of the optical output signals. The cross-connect also contains electrical means for selectively activating each switch by applying a detector voltage across the detector diodes and a modulator voltage across the modulator diodes. Preferably, the cross-connect has N rows and M columns of switches. Preferably, parallel optical waveguides are used to deliver the optical power beams to and the optical output signals from the optical switches. Each waveguide passes through a distinct row or column of the cross-connect. A second set of parallel optical waveguides may be used to deliver the optical input signals to the switches, and the two sets are substantially perpendicular to one another.